Magnetron anode having temperature compensating members within the cavities of a different coefficient of thermal expansion from the cavities



Feb. 18, 1969 v, HEATHCOTE 3,428,859

MAGNETRON ANODE HAVING TEMPERATURE COMPENSATING MEMBERS WITHIN THECAVITIES OF A DIFFERENT COEFFICIENT 0F THERMAL EXPANSION FROM THECAVITIES Filed March 15, 1966 v Fl 3. m- Henmo're- KMAQQCWLQ {$17k'DTTOKMk United States Patent 3,428,859 MAGNETRON ANODE HAVINGTEMPERATURE COMPENSATING MEMBERS WITHIN THE CAV- ITIES OF A DIFFERENTCOEFFICIENT OF THERMAL EXPANSION FROM THE CAVITIES Vincent AlbertHeathcote, London, England, assigno r t0 The M-O Valve Company, London,England, a British company Filed Mar. 15, 1966, Ser. No. 534,314 Claimspriority, application Great Britain, Mar. 23, 1965,

12,353/65 US. Cl. 315-3951 3 Claims Int. Cl. H01j 25/50 ABSTRACT OF THEDISCLOSURE -A magnetron having a multicavity anode in each of whichcavities there is disposed a member consisting of a material whosecoefiicient of thermal expansion differs from that of the material ofthe anode so as to compensate for the thermal expansion of the cavity inwhich it is disposed, thereby to reduce the magnitude of changes in thefrequency of operation of the magnetron with temperature.

This invention relates to m-agnetrons.

The invention relates particularly to magnetrons of the kind comprisingan anode in which are formed a plurality of resonant cavities which openinto a space in which is housed a cathode.

The frequency of oscillation of such a magnetron depends on the volumeof the resonant cavities; variation in the frequency of oscillation ofsuch a magnetron is thus liable to occur with changes in the temperatureof its anode.

'It is an object of the present invention to provide a magnetron of thekind specified wherein this difliculty is alleviated.

According to the present invention, in a magnetron of the kind specifiedwithin each of at least the majority of the resonant cavities there isdisposed an electrically conductive member arranged to be maintained atthe same electric potential as the anode in operation, each said memberbeing in good thermal contact with the anode and consisting of amaterial having a thermal coefi'lcient of expansion differing from thatof the material of the anode, at least over a wide range of temperaturesof the anode which may occur in normal operation of the magnetron, sothat for any change of temperature within said range the change in thefrequency of oscillation of the magnetron is less than the change in thefrequency of oscillation that would occur for that change of temperatureif said conductive members were formed of a material having the samethermal coefiicient of expansion as the material of said anode.

One arrangement in accordance with the invention will now be described,by way of example, with reference to the accompanying drawing in which:

FIGURE-1 is a part-sectional elevation of a magnetron intended foroperation in the frequency range 5,200 to 1 1,000 mc./s.;

FIGURE 2 is a view along the line II-II in FIGURE 1; and

FIGURE 3 is a perspective view of a part of the magnetron.

Referring to FIGURES -1 and 2, the magnetron has a hollow cylindricalsealed copper envelope 1 which houses an electrode structure comprisingan anode 2 and a directly heated cathode 3.

The anode 2 comprises a copper-plated tubular molybdenum member 4 ofinternal diameter 0.435 inch and is sandwiched between one end of thecurved wall of I the envelope 1 and the corresponding end of theenvelope 1. To the inner curved surface of the anode tube 4 are fixedtwelve rectangular vanes 8 which are equally spaced from one another andare also made from copper-plated molybdenum, each vane 8 having a lengthof 0.205 inch, a breadth of 0.185 inch and a thickness of 0.012 inch.The vanes 8 project radially inwards from the curved surface of theanode tube 4 so that their inner free edges lie on the surface of acylindrical volume of diameter 0.093 inch coaxial with the tube 4, theends of each vane 8 lying in the planes of the ends of the tube 4. Thecathode 3, which is tubular in shape, tits coaxially within the volumedelineated by the inner edges of the vanes 8. The copper plating uponthe tube 4 and the vanes '8 serves a dual purpose; firstly, theelectrical conductivity of the tube 4 and the vanes -8 is appreciablyincreased at the appropriate surfaces whilst, secondly, brazing of thetube 4 and the vanes '8 is facilitated.

[Extending into each vane 8 from each end are two adjoining rectangularslots 9 and 10 one of which has a width and a depth of 0.020 inch andthe other of which has a width of 0.055 inch and a depth of 0.035 inch,the deeper slot 9 being nearer the inner edge at one end of each vane '8and the shallower slot 10 being nearer the inner edge at the other endof each vane 8, and the inner edge of the inner slot at each end of eachvane 8 being spaced 0.030 inch or 0.015 inch from the inner edge of thevane 8, depending on whether the inner slot is a deeper slot 9 or ashallower slot 10. The vanes 8 are arranged so that at each end of thevanes 8 the shallower slots 10 are nearer the inner edge in alternatevanes 8, and at each end of the vanes 8 two circular metal rings 11 of0.020 inch square cross-section are arranged to run one within the innerslot in each vane 8 and the other within the outer slot in each vane 8,each ring 11 being brazed to the floor of each shallower slot 10 throughwhich it passes. Thus, each set of alternate ones of the vanes 8 iselectrically connected by the outer one of the two rings 1'1 at one end,and by the inner one of the two rings 11 at the other end this servingto suppress unwanted modes of oscillation in operation.

Referring now also to FIGURE 3, between each adjacent pair of vanes 8,except one, there is positioned a wedge-shaped copper member 12 in theform of a 30 segment of a tube of length 0.235 inch and internal andexternal diameters 0.332 inch and 0.441 inch respectively Each copperwedge 12 has each of its two rectangular faces disposed parallel to andspaced 0.014 inch from the nearest vane 8 whilst the arcuate face havingthe larger radius of curvature is spaced 0.022 inch from the interiorwall of the anode tube 4. Each copper wedge 1-2 constitutes a projectionfrom one end of a tubular copper support 13 which is mounted in goodelectrical and thermal contact with the tubular member 6 which carriesthe anode 2, thus the wedges 12 are in good electrical and thermalcontact with the anode 2.

An output is derived from the magnetron via a short section of acircular waveguide 14 sealed through the side wall of the envelope 1,the waveguide 14 having a cylindrical ceramic plug 15 sealed into it. Atits inner end the plug 15 is provided with a slot 16 into which fits anE-shaped metal coupling member 17 the central limb of which is securedto the anode tube 4 and the outer limbs of which extend through thespace between the pair of adjacent vanes 8 which are not separated by awedge 12 and are respectively connected at their ends to the outer rings11. Diametrically opposite the output waveguide a copper pinch tube 18is provided which is sealed off at its outer end during manufacture ofthe magnetron.

The spaces within the envelope 1 on each side of the anode 2 areoccupied by the two frusto-conical magnetic pole pieces 19 which areassociated with a permanent magnet (not shown) external of the envelope1 in operation of the magnetron. The narrower end of each pole piece 19is disposed adjacent the anode 2 and at its wider end, each pole piece18 is sealed through an aperture formed centrally in the correspondingend of the envelope 1. A support stem and leads 20 for the cathode 3extend through an aperture which extends coaxially through one of thepole pieces 19.

It will be appreciated that in FIGURE 2 the pole pieces 19 the cathode 3and the cathode support stem and leads 20 have been omitted for the sakeof clarity.

It will be understood that the frequency of oscillation of the magnetrondepends on the volume of the resonant cavities defined by the anode tube4, the vanes 8 and the wedges 12, the frequency of oscillationdecreasing with increase in the volume of the cavities and vice versa.It will be appreciated that the volume of the resonant cavities is thevolume contained between the vanes 8 and the anode tube 4 minus thevolume of the wedges 12. The coefficients of thermal expansion formolybdenum and copper are 5.1 l C. and l6.5 l0 C, over the range oftemperatures of the anode 2 which may occur in operation. Hence, for agiven change in temperature within the relevant range, the volume of thecopper wedges 12 changes by a larger fraction than does the volumecontained by the vanes 8 and the anode tube 4. In the magnetron,described by way of example, the relative dimensions of the wedges 12and of the anode tube 4 and the vanes 8 are chosen in relation to thedifierent thermal coefficients of expansion of the materials of whichthese members consist so that for a given change in temperature thechange in volume of the wedges 1 2 is approximately equal to the changein the volume contained by the vanes 8 and the anode tube 4. The volumeof the resonant cavities thus remains approximately constant inoperation with changes in temperature. It will be appreciated that thecopper plating on the anode tube 4 and the vanes 8 does not appreciablyatfect the thermal expansion of these members.

The particular magnetron described above, by way of example, has athermal coefficient of frequency of oscillation of about 7 kc./s./ C.; asimilar magnetron designed for operation at the same frequency andhaving a molybdenum anode but no copper wedges disposed in its resonantcavities had a thermal coefficient of frequency of oscillation of about50 kc./s./ C.

It will be appreciated that in other arrangements in accordance with theinvention materials other than molybdenum and copper may be used for theanode and electrically conductive members disposed in the resonantcavities in the anode, but of course the materials used must havesuitably diifering thermal coefiicients of expansion. It will beunderstood that the number of electrically conductive members employedis not critical. However, in order for an appreciable degree oftemperature compensation to be achieved, it is necessary for there to bea conductive member in each of at least the majority of the resonantcavities.

I claim:

1. A magnetron comprising: an anode in which is formed a plurality ofresonant cavities; a cathode housed in a space into which the resonantcavities open; and a number of electrically conductive members, each ofwhich is disposed in a different one of the resonant cavities, saidconductive members being in good electrical and thermal contact with theanode and consisting of a material having a thermal coeificient ofexpansion differing from the thermal coefiicient of expansion of thematerial of the anode at least over a wide range of temperatures of theanode which may occur in normal operation of the mag netron, so that forany change of temperature within said range, the change in the frequencyof oscillation of the magnetron, is less than the change in thefrequency of oscillation of the magnetron that would occur for thatchange in temperature if said conductive members were formed of amaterial having the same thermal coeflicient of expansion as thematerial of the anode.

2. A magnetron according to claim 1, wherein the anode consists ofmolybdenum and said conductive members consist of copper.

3. A magnetron according to claim 1, wherein the anode comprises atubular member and a plurality of planar vanes which project radiallyinwardly from the inner curved surface of said tubular member, and eachof said conductive members is in the form of a segment of a tube and isdisposed between an adjacent pair of said vanes with its two planarsides respectively parallel to the twoadjacent vanes and its curved facehaving the larger radius of curvature facing and parallel to the innercurved surface of said tubular member.

References Cited UNITED STATES PATENTS 2,810,094 10/1957 Derby et al.315-3959 X 3,289,037 11/1966 Whitmore 31539.59 X 2,520,955 9/1950 Okresset al 313311 X 2,608,673 8/1952 Brown 31539.51 X 2,811,670 10/1957Amsdem et al 31346 X 2,852,720 9/1958 Crapuchettes 313-311 X 3,297,905'1/1967 Fiedor et al. 313-311 X ELI LIEBERMAN, Primary Examiner.

S. CHATMON, In, Assistant Examiner.

US. Cl. X.R.

